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<article id="post-Redux5源码解析：store与reducer" class="h-entry article article-type-post" itemprop="blogPost" itemscope itemtype="https://schema.org/BlogPosting">
  
    
   
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        Redux5源码解析：store与reducer
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          <p>「这是我参与2022首次更文挑战的第4天，活动详情查看：<a target="_blank" rel="noopener" href="https://juejin.cn/post/7052884569032392740" title="https://juejin.cn/post/7052884569032392740">2022首次更文挑战</a>」</p>
<p>Redux可以说是一个典型的小而精的lib，源码量及api数量都不多，但设计却十分巧妙。本系列将深入Redux5源码，探究其实现与设计。</p>
<h2 id="store"><a href="#store" class="headerlink" title="store"></a>store</h2><p><code>createStore</code>用来创建store，可以接收三个参数： <code>createStore(reducer, [preloadedState], [enhancer])</code></p>
<p>preloadedState是初始state，enhancer是中间件</p>
<p>源码如下：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">function</span> <span class="title function_">createStore</span>(<span class="params"></span></span><br><span class="line"><span class="params">  reducer,</span></span><br><span class="line"><span class="params">  preloadedState?,</span></span><br><span class="line"><span class="params">  enhancer?</span></span><br><span class="line"><span class="params"></span>)&#123;</span><br><span class="line">      <span class="keyword">if</span> (<span class="keyword">typeof</span> enhancer !== <span class="string">&#x27;undefined&#x27;</span>) &#123;</span><br><span class="line"></span><br><span class="line">        <span class="keyword">return</span> <span class="title function_">enhancer</span>(createStore)(</span><br><span class="line">          reducer,</span><br><span class="line">          preloadedState</span><br><span class="line">        )</span><br><span class="line">      &#125;</span><br><span class="line">     ...</span><br><span class="line">     <span class="keyword">const</span> store = &#123;</span><br><span class="line">        <span class="attr">dispatch</span>: dispatch <span class="keyword">as</span> <span class="title class_">Dispatch</span>&lt;A&gt;,</span><br><span class="line">        subscribe,</span><br><span class="line">        getState,</span><br><span class="line">        replaceReducer,</span><br><span class="line">        [$$observable]: observable</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">return</span> store</span><br><span class="line"></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>store就是一个由dispatch、subscribe、 getState和 replaceReducer组成的对象</p>
<h3 id="getState与currentState"><a href="#getState与currentState" class="headerlink" title="getState与currentState"></a>getState与currentState</h3><p>Redux将state维护在currentState变量中，并提供getState获取state，禁止直接获取</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> currentState = preloadedState </span><br><span class="line">...</span><br><span class="line"><span class="keyword">function</span> <span class="title function_">getState</span>(<span class="params"></span>) &#123;</span><br><span class="line">   <span class="keyword">if</span> (isDispatching) &#123;</span><br><span class="line">     <span class="keyword">throw</span> <span class="keyword">new</span> <span class="title class_">Error</span>(<span class="comment">/*dispatching时禁止读取 */</span>)</span><br><span class="line">   &#125;</span><br><span class="line"></span><br><span class="line">   <span class="keyword">return</span> currentState </span><br><span class="line">&#125;</span><br><span class="line"></span><br></pre></td></tr></table></figure>

<h3 id="reducer、-dispatch与subscribe"><a href="#reducer、-dispatch与subscribe" class="headerlink" title="reducer、 dispatch与subscribe"></a>reducer、 dispatch与subscribe</h3><p>dispatch的时候会将state与action交给reducer处理，并将currentState更新问reducer返回的state。之后调用通过subscribe注册的listener</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> currentReducer = reducer</span><br><span class="line"><span class="keyword">function</span> <span class="title function_">subscribe</span>(<span class="params">listener</span>) &#123;</span><br><span class="line">   </span><br><span class="line">    <span class="keyword">let</span> isSubscribed = <span class="literal">true</span></span><br><span class="line"></span><br><span class="line">    <span class="title function_">ensureCanMutateNextListeners</span>()</span><br><span class="line">    nextListeners.<span class="title function_">push</span>(listener)</span><br><span class="line"></span><br><span class="line">    <span class="keyword">return</span> <span class="keyword">function</span> <span class="title function_">unsubscribe</span>(<span class="params"></span>) &#123; ...&#125;</span><br><span class="line"> &#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">function</span> <span class="title function_">dispatch</span>(<span class="params">action: A</span>) &#123;</span><br><span class="line">    </span><br><span class="line">    <span class="keyword">try</span> &#123;</span><br><span class="line">      isDispatching = <span class="literal">true</span></span><br><span class="line">      currentState = <span class="title function_">currentReducer</span>(currentState, action)</span><br><span class="line">    &#125; <span class="keyword">finally</span> &#123;</span><br><span class="line">      isDispatching = <span class="literal">false</span></span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">const</span> listeners = (currentListeners = nextListeners)</span><br><span class="line">    <span class="keyword">for</span> (<span class="keyword">let</span> i = <span class="number">0</span>; i &lt; listeners.<span class="property">length</span>; i++) &#123;</span><br><span class="line">      <span class="keyword">const</span> listener = listeners[i]</span><br><span class="line">      <span class="title function_">listener</span>()</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">return</span> action</span><br><span class="line">  &#125;</span><br></pre></td></tr></table></figure>

<h3 id="其他属性"><a href="#其他属性" class="headerlink" title="其他属性"></a>其他属性</h3><ul>
<li><code>replaceReducer</code>:支持动态更换reducer</li>
<li><code>[$$observable]: observable</code>可以将Redux与其他observable&#x2F;reactive的lib搭配</li>
</ul>
<h2 id="reducer"><a href="#reducer" class="headerlink" title="reducer"></a>reducer</h2><p>reducer是一个接收action和state，并返回一个新的state的函数。除了createStore，与之相关的另一个顶层api是<code>combineReducers</code>。虽然Redux不支持多store,但通过这个api，我们能够以namespace切分state。</p>
<p>用法如下：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// reducers/todos.js</span></span><br><span class="line"><span class="keyword">export</span> <span class="keyword">default</span> <span class="keyword">function</span> <span class="title function_">todos</span>(<span class="params">state = [], action</span>) &#123;</span><br><span class="line">  <span class="keyword">switch</span> (action.<span class="property">type</span>) &#123;</span><br><span class="line">    <span class="keyword">case</span> <span class="string">&#x27;ADD_TODO&#x27;</span>:</span><br><span class="line">      <span class="keyword">return</span> state.<span class="title function_">concat</span>([action.<span class="property">text</span>])</span><br><span class="line">    <span class="attr">default</span>:</span><br><span class="line">      <span class="keyword">return</span> state</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">// reducers/counter.js</span></span><br><span class="line"><span class="keyword">export</span> <span class="keyword">default</span> <span class="keyword">function</span> <span class="title function_">counter</span>(<span class="params">state = <span class="number">0</span>, action</span>) &#123;</span><br><span class="line">  <span class="keyword">switch</span> (action.<span class="property">type</span>) &#123;</span><br><span class="line">    <span class="keyword">case</span> <span class="string">&#x27;INCREMENT&#x27;</span>:</span><br><span class="line">      <span class="keyword">return</span> state + <span class="number">1</span></span><br><span class="line">    <span class="keyword">case</span> <span class="string">&#x27;DECREMENT&#x27;</span>:</span><br><span class="line">      <span class="keyword">return</span> state - <span class="number">1</span></span><br><span class="line">    <span class="attr">default</span>:</span><br><span class="line">      <span class="keyword">return</span> state#### <span class="string">`reducers/index.js`</span></span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">// reducers/index.js</span></span><br><span class="line"><span class="keyword">import</span> &#123; combineReducers &#125; <span class="keyword">from</span> <span class="string">&#x27;redux&#x27;</span></span><br><span class="line"><span class="keyword">import</span> todos <span class="keyword">from</span> <span class="string">&#x27;./todos&#x27;</span></span><br><span class="line"><span class="keyword">import</span> counter <span class="keyword">from</span> <span class="string">&#x27;./counter&#x27;</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">export</span> <span class="keyword">default</span> <span class="title function_">combineReducers</span>(&#123;</span><br><span class="line">  todos,</span><br><span class="line">  counter</span><br><span class="line">&#125;)</span><br><span class="line"><span class="comment">// App.js</span></span><br><span class="line"><span class="keyword">import</span> &#123; createStore &#125; <span class="keyword">from</span> <span class="string">&#x27;redux&#x27;</span></span><br><span class="line"><span class="keyword">import</span> reducer <span class="keyword">from</span> <span class="string">&#x27;./reducers/index&#x27;</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> store = <span class="title function_">createStore</span>(reducer)</span><br><span class="line"><span class="variable language_">console</span>.<span class="title function_">log</span>(store.<span class="title function_">getState</span>())</span><br><span class="line"><span class="comment">// &#123;</span></span><br><span class="line"><span class="comment">// counter: 0,</span></span><br><span class="line"><span class="comment">// todos: []</span></span><br><span class="line"><span class="comment">// &#125;</span></span><br></pre></td></tr></table></figure>

<h3 id="核心源码"><a href="#核心源码" class="headerlink" title="核心源码"></a>核心源码</h3><figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">export</span> <span class="keyword">default</span> <span class="keyword">function</span> <span class="title function_">combineReducers</span>(<span class="params">reducers: ReducersMapObject</span>) &#123;</span><br><span class="line">  ...</span><br><span class="line">  <span class="keyword">return</span> <span class="keyword">function</span> <span class="title function_">combination</span>(<span class="params"></span></span><br><span class="line"><span class="params">    state: StateFromReducersMapObject&lt;<span class="keyword">typeof</span> reducers&gt; = &#123;&#125;,</span></span><br><span class="line"><span class="params">    action: AnyAction</span></span><br><span class="line"><span class="params">  </span>) &#123;</span><br><span class="line">    <span class="keyword">let</span> hasChanged = <span class="literal">false</span></span><br><span class="line">    <span class="keyword">const</span> <span class="attr">nextState</span>: <span class="title class_">StateFromReducersMapObject</span>&lt;<span class="keyword">typeof</span> reducers&gt; = &#123;&#125;</span><br><span class="line">    <span class="comment">// 这里调用了所有的reducer</span></span><br><span class="line">    <span class="keyword">for</span> (<span class="keyword">let</span> i = <span class="number">0</span>; i &lt; finalReducerKeys.<span class="property">length</span>; i++) &#123;</span><br><span class="line">      <span class="keyword">const</span> key = finalReducerKeys[i]</span><br><span class="line">      <span class="keyword">const</span> reducer = finalReducers[key]</span><br><span class="line">      <span class="keyword">const</span> previousStateForKey = state[key]</span><br><span class="line">      <span class="keyword">const</span> nextStateForKey = <span class="title function_">reducer</span>(previousStateForKey, action)</span><br><span class="line">     </span><br><span class="line">      nextState[key] = nextStateForKey</span><br><span class="line">      hasChanged = hasChanged || nextStateForKey !== previousStateForKey</span><br><span class="line">    &#125;</span><br><span class="line">    hasChanged =</span><br><span class="line">      hasChanged || finalReducerKeys.<span class="property">length</span> !== <span class="title class_">Object</span>.<span class="title function_">keys</span>(state).<span class="property">length</span></span><br><span class="line">    <span class="keyword">return</span> hasChanged ? nextState : state</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br></pre></td></tr></table></figure>

<p><strong>从源码看出这里有个坑：</strong></p>
<p>当dispatch的时候，Redux并不能判断应该由哪个reducer执行，因此一股脑让所有的reducer都执行了一遍。通过判断返回的state与原本的state是否相同，确定是返回原先的state还是新的state。这样可以避免触发多余的listener。</p>
<p><strong>因此reducer中如果没有action匹配，那么务必要<code>return state</code>,返回原本的state,而不能返回一个对象字面量或新的state</strong>。</p>
<h2 id="bindActionCreators"><a href="#bindActionCreators" class="headerlink" title="bindActionCreators"></a>bindActionCreators</h2><p>这个api用来简化dispatch和action，可以理解为柯里化。</p>
<p>比如组件需要进行这样一个调用：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="title function_">dispatch</span>(&#123;</span><br><span class="line">    <span class="attr">type</span>: <span class="string">&#x27;REMOVE_TODO&#x27;</span>,</span><br><span class="line">    id</span><br><span class="line">&#125;)</span><br></pre></td></tr></table></figure>

<p>那么这个组件既要能够获取到dispatch，又要知道action的类型，但这些往往都不是子组件应该关心的。</p>
<p>这时候就可以使用bindActionCreators，将action和dispatch封装起来。</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> <span class="title class_">TodoActionCreators</span> = &#123;</span><br><span class="line">     <span class="title function_">removeTodo</span>(<span class="params">id</span>) &#123;</span><br><span class="line">          <span class="keyword">return</span> &#123;</span><br><span class="line">                <span class="attr">type</span>: <span class="string">&#x27;REMOVE_TODO&#x27;</span>,</span><br><span class="line">                id</span><br><span class="line">          &#125;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"> <span class="keyword">const</span> &#123; removeTodo &#125; = <span class="title function_">useMemo</span>(</span><br><span class="line">    <span class="function">() =&gt;</span> <span class="title function_">bindActionCreators</span>(<span class="title class_">TodoActionCreators</span>, dispatch),</span><br><span class="line">    [dispatch]</span><br><span class="line">  )</span><br></pre></td></tr></table></figure>

<h3 id="核心源码-1"><a href="#核心源码-1" class="headerlink" title="核心源码"></a>核心源码</h3><figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">function</span> bindActionCreator&lt;A <span class="keyword">extends</span> <span class="title class_">AnyAction</span> = <span class="title class_">AnyAction</span>&gt;(</span><br><span class="line">  <span class="attr">actionCreator</span>: <span class="title class_">ActionCreator</span>&lt;A&gt;,</span><br><span class="line">  <span class="attr">dispatch</span>: <span class="title class_">Dispatch</span></span><br><span class="line">) &#123;</span><br><span class="line">  <span class="keyword">return</span> <span class="keyword">function</span> (<span class="params"><span class="variable language_">this</span>: any, ...args: any[]</span>) &#123;</span><br><span class="line">      <span class="comment">//核心源码就这一句</span></span><br><span class="line">    <span class="keyword">return</span> <span class="title function_">dispatch</span>(actionCreator.<span class="title function_">apply</span>(<span class="variable language_">this</span>, args))</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">export</span> <span class="keyword">default</span> <span class="keyword">function</span> <span class="title function_">bindActionCreators</span>(<span class="params"></span></span><br><span class="line"><span class="params">  actionCreators: ActionCreator&lt;any&gt; | ActionCreatorsMapObject,</span></span><br><span class="line"><span class="params">  dispatch: Dispatch</span></span><br><span class="line"><span class="params"></span>) &#123;</span><br><span class="line">  <span class="keyword">if</span> (<span class="keyword">typeof</span> actionCreators === <span class="string">&#x27;function&#x27;</span>) &#123;</span><br><span class="line">    <span class="keyword">return</span> <span class="title function_">bindActionCreator</span>(actionCreators, dispatch)</span><br><span class="line">  &#125;</span><br><span class="line"></span><br><span class="line">  <span class="keyword">const</span> <span class="attr">boundActionCreators</span>: <span class="title class_">ActionCreatorsMapObject</span> = &#123;&#125;</span><br><span class="line">  <span class="keyword">for</span> (<span class="keyword">const</span> key <span class="keyword">in</span> actionCreators) &#123;</span><br><span class="line">    <span class="keyword">const</span> actionCreator = actionCreators[key]</span><br><span class="line">    <span class="keyword">if</span> (<span class="keyword">typeof</span> actionCreator === <span class="string">&#x27;function&#x27;</span>) &#123;</span><br><span class="line">      boundActionCreators[key] = <span class="title function_">bindActionCreator</span>(actionCreator, dispatch)</span><br><span class="line">    &#125;</span><br><span class="line">  &#125;</span><br><span class="line">  <span class="keyword">return</span> boundActionCreators</span><br><span class="line">&#125;</span><br><span class="line"></span><br></pre></td></tr></table></figure>

<p>里面的核心就一行<code>dispatch(actionCreator.apply(this, args))</code>。</p>
<h3 id="react-redux"><a href="#react-redux" class="headerlink" title="react-redux"></a>react-redux</h3><p>Redux本身只提供了store的创建和管理，后续系列将从react-redux源码看它与react是如何结合的。</p>
<p>未完待续~</p>

        
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